Rotary anode X-ray tube apparatuses are used in medical and industrial diagnosis systems that are represented by computed tomography (CT) apparatuses. In general, a rotary anode X-ray tube apparatus includes a rotary anode X-ray tube that radiates X-rays, a stator coil, and a casing that stores the rotary anode X-ray tube and the stator coil.
A conventional rotary anode X-ray tube includes a stationary shaft that has a flange portion provided in a portion thereof, a rotary anode that is rotatably provided in the stationary shaft, a cathode that is disposed to face the rotary anode, and a vacuum enclosure that stores the stationary shaft, the rotary anode, and the cathode and partially has a transmissive window. The rotary anode X-ray tube has a cantilevered structure where the stationary shaft is supported to one side of the vacuum enclosure.
The rotary anode includes a rotary cylinder of a cylindrical shape having a bottom that is provided to cover a portion from a front end of the stationary shaft to the flange portion with a gap, a target (anode) of a hollow circular plate shape that is provided on a front end of the rotary cylinder, a motor rotor that is provided on a side of the rotary cylinder, and a thrust ring that is provided in an opening of the rotary cylinder. The target and the rotary cylinder may be separated components.
A liquid metal is filled into a gap of the stationary shaft and the rotary cylinder. The liquid metal works as a lubricant of the hydrodynamic bearing when the rotary anode rotates. The rotary anode is rotatably supported due to a hydrodynamic effect that is generated in the hydrodynamic bearing.
The hydrodynamic bearing includes a radial bearing that supports the rotary anode in a radial direction and a thrust bearing that supports the rotary anode in an axial direction. In portions that constitute the radial bearing and the thrust bearing, grooves to generate the hydrodynamic effect are provided.
In the conventional rotary anode X-ray tube apparatus, the rotary anode is rotated by generating a magnetic field in the motor rotor by the stator coil. In this state, electronic beams are irradiated from the cathode to the target. If electrons collide with the target, X-rays are discharged from a transmissive window provided in the vacuum enclosure to the outside.
When the electronic beams are irradiated onto the target, the X-rays are generated as described above, but heat is also simultaneously generated. Accordingly, the temperature of the target, particularly on an electron colliding surface (focus) where the electrons collide, becomes locally high. The electron colliding surface is instantly destroyed due to the impact of the locally raised temperature of the surface, that is thermal shock. For this reason, in the conventional rotary anode X-ray tube apparatus, the heat input to the target is dispersed by rotating the target. Dispersing the heat input prevent the electron colliding surface from being severely damaged. As such, the X-ray tube that rotates the target (anode) is called the rotary anode X-ray tube. In the description below, the rotary anode X-ray tube is simply called the X-ray tube.
Even in the X-ray tube where the target rotates, if the electronic beams are continuously irradiated for a certain amount of time, the heat is accumulated in the target. If the heat accumulated in the target exceeds the heat capacity of the target, the temperature of the electron colliding surface gradually increases. If the temperature of the electron colliding surface exceeds its allowable temperature, the surface of the target starts to be damaged. This problem is resolved by increasing a size of the target and increasing the heat capacity. However, as the size of the target increases, the size of the X-ray tube increases, leading to higher weight and manufacturing cost of the X-ray tube. Accordingly, the method that increases the size of the target to resolve the above problem is not a preferable method.
For this reason, development of a technology for cooling the target is being promoted. In the cooling of the target, cooling based on radiation or cooling based on heat transfer is used. In particular, the cooling based on the heat transfer is a method that transmits the heat generated in the rotary anode to the stationary shaft through the liquid metal serving as the lubricant of the hydrodynamic bearing and removes the heat transmitted to the stationary shaft by a cooling fluid flowing in the stationary shaft. In the method that cools the target through the heat transfer, higher cooling efficiency can be obtained as compared with the cooling of the target through the radiation. Accordingly, in recent years, X-ray tubes that can cool the target through the heat transfer are mainly used.
The X-ray tube described above has the cantilevered structure where the stationary shaft is supported to one side of the vacuum enclosure. However, an X-ray tube having a both-end supported structure where the stationary shaft is supported to both facing sides of the vacuum enclosure is also known. Unlike the X-ray tube having the cantilevered structure, the X-ray tube having the both-end supported structure can flow the cooling fluid in one direction. Accordingly, as compared with the X-ray tube having the cantilevered structure, the X-ray tube having the both-end supported structure needs a smaller flow passage diameter in flowing a cooling fluid of the same flow volume, and therefore, has an advantage that bending rigidity of the stationary shaft can be enhanced. Since the stationary shaft is supported to both ends, the stationary shaft is rarely bent and deformed, even when a high load is applied to the stationary shaft.
In the conventional X-ray tube described above, the rotary anode is supported by the hydrodynamic effect. In order to generate the hydrodynamic effect, the stationary shaft and the rotary cylinder that constitute the bearing need to be disposed close to each other. However, if the temperature of the rotary cylinder becomes high due to a raised temperature of the target, the rotary cylinder thermally expands and the gap of the hydrodynamic bearing, that is, the gap of the stationary shaft and the rotary cylinder is enlarged. As a result, if the temperature of the rotary cylinder becomes high, a load carrying capacity of the bearing is lowered and a normal rotational motion is disabled. Accordingly, a widely-known configuration includes, in some places of the stationary shaft and the rotary cylinder constituting the bearing, stationary shaft and the rotary cylinder that are provided close to each other and a heat-transfer unit that is configured by filling the liquid metal between the stationary shaft and the rotary cylinder is provided; and, in places other than the bearing, the gap of the stationary shaft and the rotary cylinder that is enlarged. According to this configuration, the heat transfer to the hydrodynamic bearing is prevented and the heat of the target is transmitted to a cooling fluid in the stationary shaft through the liquid metal interposed in the heat-transfer unit provided separately from the hydrodynamic bearing. As such, since the heat transfer is mainly made in the heat-transfer unit, the gap of the hydrodynamic bearing is suppressed from being enlarged. Accordingly, the load carrying capacity of the hydrodynamic bearing can be suppressed from being lowered.
Meanwhile, in the CT apparatus where the X-ray tube apparatus is mounted, a helical scanning scheme that irradiates X-rays onto an inspected object, such as a person, while revolving the X-ray tube apparatus around the inspected object is adopted. The helical scanning speed, that is, the revolving speed of the X-ray tube apparatus has been increased. Due to the increase in the helical scanning speed, it is required to improve anti-G performance of the X-ray tube that is mounted in the X-ray tube apparatus. In this case, the improvement of the anti-G performance means that the stationary shaft is rarely bent and deformed even when the X-ray tube receives the centrifugal force due to the high-speed scanning. When the anti-G performance of the X-ray tube is low, if the stationary shaft supports the rotary anode where the centrifugal force is applied, the stationary shaft is bent and deformed. If the rotary anode and the stationary shaft that constitute the bearing are relatively greatly inclined due to the bending deformation of the stationary shaft, the rotary anode and the stationary shaft contact at an end of the bearing and superior rotation stability can not be obtained. Accordingly, if the anti-G performance is improved, the rotary anode and the stationary shaft can be suppressed from contacting at the end of the bearing and superior rotation stability of the rotary anode can be keeped.
The improvement of the anti-G performance of the X-ray tube is achieved by improving the bending rigidity of the stationary shaft.
Meanwhile, the X-ray tube is also required to have a high output of the X-rays. However, amount of heat generated in the target increases with the high output. Accordingly, due to the high output, a function of cooling the target with high heat transfer efficiency is needed. In the conventional X-ray tube, the wall thickness of the entire stationary shaft may be thinned to improve the heat transfer efficiency. However, if the wall thickness of the stationary shaft is decreased, sufficient anti-G performance is not obtained due to lowered rigidity of the stationary shaft.
That is, in the conventional X-ray tube, it is difficult to simultaneously realize the improvement of the anti-G performance and the high output.
In the conventional X-ray tube where the heat-transfer unit is separately provided, the liquid metal is interposed in the heat-transfer unit in addition to the bearing. For this reason, an area where viscosity friction of the liquid metal is generated increases, leading to increased frictional loss. Accordingly, in order to compensate for the frictional loss and rotate the anode at a high speed, rotation torque of the motor needs to be increased. Since the rotation toque of the motor is determined by strength of the magnetic field generated by the stator coil, the size of the stator coil needs to be increased to generate the stronger magnetic field. Since the size of the X-ray tube apparatus including the X-ray tube increases due to the increase in the size of the stator coil, the weight of the X-ray tube apparatus greatly increases. As the helical scanning speed of the CT apparatus increases, it is required to decrease the weight of apparatuses mounted on mount of the CT apparatus, and it is increasingly required to decrease the size and the weight of the X-ray tube apparatus. Accordingly, the increase in the size and the weight of the X-ray tube apparatus becomes a problem.
A total amount of heat generation of a rotating mechanism increases due to the frictional heat of the liquid metal interposed in the heat-transfer unit. Accordingly, it is required to decrease the demand output of the X-rays or further include a cooling mechanism that works by thinning the wall thickness of the heat transmitting part. That is, it is further difficult to simultaneously realize the improvement of the anti-G performance and the high output.
In the heat-transfer unit that does not have a groove to draw the lubricant in the bearing and keep the lubricant, like the hydrodynamic bearing, the liquid metal serving as a heat-transfer material may not exist in an assumed heat transfer area. Accordingly, cooling performance may not be constant and reliability of the cooling performance is low.
As described above, in the conventional X-ray tube, it is difficult to simultaneously realize the improvement of the anti-G performance and the high output. In the conventional X-ray tube where the heat-transfer unit is provided separately from the hydrodynamic bearing, the size of the X-ray tube apparatus including the X-ray tube increases and the weight increases.